Thermistor and method of manufacturing same

ABSTRACT

A thermistor comprising a monocrystalline semiconductor substrate having at a surface thereof both first and second electrically energized electrodes and means for providing a higher current density at the central region of this surface than at the edge of this surface and at the other surfaces of the substrate, which means comprises at least one electricallyfloating field-conducting electrode between the first and second electrode. In one embodiment, the field-conducting electrode is spaced from the edge of the surface on which it is located. In another embodiment, the thermistor also includes a conducting edge limitation.

United States Patent 1 Bethe THERMISTOR AND METHOD OF MANUFACTURING SAME [75] inventor: Klaus Bethe, Ellerbek, Germany [73] Assignee: U.S. Philips Corporation, New York,

[22] Filed: Dec. 16, 1971 [21] Appl. No.: 208,566

[30] Foreign Application Priority Data Jan. 8, l971 Germany P 21 00 789.6

[52] US. Cl. 338/22, 338/322 [51] Int. Cl 1101c 7/04 [58] Field of Search 338/13, 20, 22-25, 338/322, 324-326, 327-329; 29/612, 621, 592; 317/235 Q [56] References Cited UNITED STATES PATENTS 3,343,114 9/1967 Rice 338/22 R [451 July 10,1973

' 2,075,733 3/1937 Lazarus 338/20 3,097,336 7/1963 Sziklai et al. 338/l3 X Primary Examiner-C. L. Albritton Attorney-Frank R. Trifari [5 7 ABSTRACT A thermistor comprising a monocrystalline semiconductor substrate having at a surface thereof both first and second electrically energized electrodes and means for providing a higher current density at the central region of this surface than at the edge of this surface and at the other surfaces of the substrate, which means comprises at least one electrically-floating fieldconducting electrode between the first and second electrode. In one embodiment, the field-conducting electrode is spaced from the edge of the surface on which it is located. in another embodiment, the thermistor also includes a conducting edge limitation.

15 Claims, 4 Drawing Figures THERMISTOR AND METHOD OF MANUFACTURING SAME The invention relates to a thermistor of monocrystalline semiconductor material and having at least two electrodes provided on a main surface of the semiconductor body, and also to a method of manufacturing such a thermistor.

Known semiconductor thermistors consist of polycrystalline sintered bodies of different metal oxides these semiconductor thermistors serving as temperature-dependent resistors for the electrical measurement of temperature. In comparison with metallic resistance thermometers, they are characterized by higher sensitivity (approximately 4 per C versus 0.4 percent per C), small dimensions, higher resistance and lower manufacturing costs. As regards manufacturing tolerances (resistance, sensitivity) and stability, these polycrystalline thermistors with sintered-in electrodes, can only satisfy requirements which are not very severe.

It is also known (United States Pat. No. 3,270,309) to use intrinsic monocrystalline germanium as a material for thermistors. In such thermistors, the absolute value and the thermal behaviour of the conductivity are accurately defined by physical laws.

So far, however, it has not been possible to manufacture germanium thermistors which are stable over a prolonged period of time and have very narrow tolerances, as the spread in the resistance-determining dimensions of the semiconductor body and the electrodes is too large during manufacture. This difficulty has not been overcome either by a further known thermistor (DT.0.S. 1804012) having the electrodes provided on one main surface of a monocrystalline semiconductor body, as the electrodes are situated at the edges of the semiconductor body and, consequently fluctuations in the external dimensions of the semiconductor body -determined only coarsely by scratching and breaking during manufactureinfluence the electrode geometry determining the electrical values.

The invention has for its object to eliminate these drawbacks of the known thermistors. This object is fulfilled according to the invention in that the shape, the size and the mutual distance of the electrodes is chosen to be such that the current density in the semiconductor body at the edge of the main surface and on all other surfaces is very slight in comparison with the current density at the centre of the main surface.

In order to keep the current density in the semiconductor body very low at the edge of the main surface and on all other surfaces, additional non-fed field conducting electrodes influencing the current density may be arranged between-both fed electrodes. The main surface of the semiconductor body may furthermore be provided with a conducting edge limitation. The semiconductor body preferably consists of intrinsic germanium.

Particularly efficient methods of manufacturing such thermistors are characterized in that the electrodes are provided on the main surface of the semiconductor body, either by photolithographic techniques or by the silk-screening technique.

The advantages achieved by the invention, particularly consist in that the electrical resistance measured between both electrodes is determined substantially exlusively by the geometry of the electrodes (i.e. their dimensions and their distance from each other), whilst the effect of the outside dimensions of the semiconductor body on the resistance is negligible. Furthermore, the susceptibility to contamination of the other surfaces of the semiconductor body is greatly reduced as a result of the field concentration within the range of the electrodes. The planar arrangement of the electrodes permits the use of either the high-precision photolithographic techniques or the silk-screening technique for their manufacture, so that the geometrical magnitudes (dimensions and distance of the electrodes) determining the electrical resistance of the thermistor can be very accurately maintained. These techniques have given very favourable results in bulk manufacture.

The further requirement for accurately manintaining the electrical resistance of the thermistor, i.e. accurately defined material properties of the semiconductor material, is satisfied by intrinsic semiconductor single crystals. For the temperature range of from 40 to +300 C germanium is suitable, whilst for lower temperatures InSb and for temperature exceeding C silicon is to be preferred.

In order that the invention may be readily carried into effect, three embodiments thereof will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIG. 1 is a perspective view of a first embodiment of a thermistor according to the invention,

FIG. 2 is a plan view of the semiconductor body of a second embodiment of a thermistor according to the invention, having two field conducting electrodes arranged between the main electrodes.

FIG. 2a is a cross-sectional view through the semiconductor body shown in FIG. 2, and

FIG. 3 is a plan view of the semiconductor body of a third embodiment of a thermistor according to the invention, having an electrode arrangement which substantially corresponds to the embodiment shown in FIG. 2, and a conducting edge limitation on the main surface of the semiconductor body accommodating the electrodes.

FIG. 1 shows a first embodiment of a thermistor according to the invention, consisting of a parallelepiped body 1 of intrinsic monocrystalline germanium. Two circular metal electrodes 2 of a gold/antimony alloy are alloyed with the main surface of the semiconductor body. Two gold connecting wires 3 are secured to these electrodes by pressure welding. The main surface of the semiconductor body accommodating the two electrodes 2 is protected by an insulating layer 4. The lower side of the semiconductor body, over which the heat to be measured is fed to the thermistor, is covered by a further thin insulating layer 5 In a thermistor thus constructed the current density has its maximum value on the direct connecting line between the two circular electrodes 2, and it decreases very rapidly to negligibly small values near the edge of the main surface. Consequently, the resistance measured between both electrodes 2, is extremely sensitive to fluctuations in the outside dimensions of the semiconductor body (a, b, c).

In a practical embodiment of the thermistor thus con structed and having thedimensions d 0.1 mm

r 0.2 mm,

the following values are obtained.

A a/a= IO 1A R /R c z 1%.

A b/b= l0 :A R /R C 1%.

A c/c 3 :A R /R o C 1 As the thickness c of the semiconductor body can still be maintained comparatively accurately due to the lapping and etching technique followed during manufacture, a A do of 3 percent could be taken into account, whilst for the side lengths a and b deviations of percent had to be taken into account due to the scratching and breaking techniques used for manufacturing the wafers.

Using these dimensions of the semiconductor body an electrical resistance of the thermistors of 1.6 k.ohm i 1 percent is obtained at C. The measuring range lies between 40 and +300 C. At a load of 0.8 mW self-heating amounting to 1 C occurs.

In the case of circular electrodes a strong field concentration is produced, which imposes an upper limit as regards the electrical loadability of the thermistor. An increase of the electrode diameter, however, causes an unwanted resistance decrease and also, due to the deeper penetration of the electrical flow field, an increased effect of possible fluctuations in the thickness of the semiconductor body.

Consequently for higher electrical loads a thermistor constructed according to FIG. 2 is better suitable. Even though a comparatively large part of the volume of the semiconductor body is electrically loaded, the effect of the dimensions of this body on the electrical resistance is not increased in comparison with the embodiment shown in FIG. 1.

The thermistor shown in FIG. 2 also consists of a body of intrinsic, monocrystalline semiconductor mateiral, on the main surface of which two bar-shaped metal electrodes 2 are arranged which make. contact via connecting wires 3. Between these fed electrodes 2 there are situated two likewise bar-shaped fieldconducting electrodes 6 which increase the current density in the surface range of the semiconductor body, thus keeping the effect of the thickness of the semiconductor body small, in spite of the large distance between the fed electrodes 2. The cross-sectional view through the semiconductor body of FIG. 2a shows the current lines i and the potential lines 4: to illustrate the effect. In order to obtain a field distribution which is as uniform as possible, the distance between both fieldconducting electrodes 6 perferably exceeds slightly the distance of the field conducting electrodes from the fed electrodes 2. Consequently, the thermistor consists substantially of a series connection of three single resistors between the two bar-shaped fed electrodes.

The principle of the field-conducting electrodes can, of course, also be applied to electrodes of another shape, for example, to the annular electrodes shown in FIG. 1. An arbitrary number of field-conducting electrodes may be chosen.

FIG. 3 shows a third embodiment of a thermistor according to the invention where the main surface of an intrinsic, monocrystalline semiconductor body according to the embodiment shown in FIG. 2, accommodates two bar-shaped electrodes 2, which are contacted and fed via connecting wires 3, two bar-shaped fieldconducting electrodes 6 being provided between the former two electrodes 2. In addition, however, the main surface of the semiconductor body of this embodiment accommodates a conducting edge limitation, which, however, is not electrically contacted. This edge limitation 7 carries an additional current i2 parallel to the current that is flowing between the electrodes 2, so

that the distribution of the electrical energy is further improved. Since the electrical resistance of the path for the current i2 is essentially determined by the distance between the fed electrodes 2 and the inner limitation of the conducting edge limitation and since it is possible to maintain this distance accurately in manufacture, the width of the edge limitation, which depends on the spread of the outside dimensions of the semiconductor body, has no essential effect on the overall resistance. This edge limitation has a further advantage in that the scratching by means of a diamond required for separating the individual semiconductor bodies from each other, need not penetrate the tough insulating layer 4 (e.g. an Si0 -layer). For the embodiment shown in FIG. 1 and 2, this can be achieved only-by means of an additional manufacturing step involving the removal of the insulating layer at the area of the scratch lines by etch- In the manufacture of the described thermistors, the electrodes and optionally the edge limitation are provided on the main surface of the semiconductor body by the methods commonly applied for bulk manufacture of semiconductor structural elements. This is mainly vapour deposition and the use of photolithographically made masks. First the main surface of all (still cohering) semiconductor bodies are covered by a suitable insulating layer, for example, of Si0 The parts of the insulating layer surfaces, where the semiconductor body is to be covered with metal in order to form the electrodes and optionally the edge limitation, are etched after a photo resist mask has been provided. When this has been done, the metal, for example, gold is vapour deposited and possibly alloyed into the surface of the semiconductor body.

However, under circumstances it is alternatively possible to provide the metal layers constituting the electrodes and optionally the edge limitation on the main surfaces of the semiconductor body by the silkscreening technique.

I claim:

1. A thermistor comprising a. A monocrystalline semiconductor body having a first surface;

b. at least first and second electrodes located on and electrically connected to said first surface;

c. means for applying electrical potentials to said first and second electrodes; and

d. means for reducing the density of said current in said semiconductor body at the edges of said first surface and at all other surfaces of said body compared with the density of said current at the central regions of said first surface when said potentials are applied to said first and second electrodes, said means comprising at least one electrically floating field conducting electrode located between said first and second electrodes, said field conducting electrode being located on and electrically connected to said first surface.

2. A thermistor as defined in claim 1, wherein said field conducting electrode is spaced from the edges of said first surface.

3. A thermistor as recited in claim 2, wherein said first and second electrodes and said field conducting electrode are of different sizes and shapes.

4. A thermistor as recited in claim 2, wherein said first and second electrodes and said field conducting electrode are substantially bar-shaped and said first and second electrodes are longer than said field conducting electrode.

5. A thermistor as recited in claim 2, wherein said first and second electrodes are located at opposite edges of said first surface.

6. A thermistor as recited in claim 5, wherein said first surface is substantially rectangular and each of said first and second electrodes extends to the other opposite edges of said first surface.

7. A thermistor as recited in claim 2, comprising a plurality of said field conducting electrodes located between said first and second electrodes, wherein the distance between a field conducting electrode and one of said first and second electrodes is substantially different from the distance between adjacent ones of said field conducting electrodes.

8. A thermistor as recited in claim 7, wherein the first mentioned distance is less than the second mentioned distance.

9. A thermistor as recited in claim 2, wherein said first and second electrodes are substantially circular in shape.

10. A thermistor as recited in claim 2, further comprising a conducting edge limitation located at said first surface.

11. A thermistor as recited in claim 10, wherein said edge limitation extends substantially completely around the edges of said first surface.

12. A thermistor as recited in claim 2, further comprising a protective layer at the parts of said first surface not covered by said first and second electrodes and said field conducting electrode.

13. A thermistor as recited in claim 12, wherein said protective layer consists essentially of silicon dioxide.

14. A thermistor as recited in claim 12, wherein said first surface comprises a major surface of said semiconductor body, said body further comprising a second major surface opposite said first surface and a second protective layer covering said second major surface, said second layer consisting essentially of one of silicon dioxide and aluminum oxide.

15. A thermistor as recited in claim 2, wherein said semiconductor body consists essentially of one of intrinsic germanium, silicon, and indium-antimonide. 

2. A thermistor as defined in claim 1, wherein said field conducting electrode is spaced from the edges of said first surface.
 3. A thermistor as recited in claim 2, wherein said first and second electrodes and said field conducting electrode are of different sizes and shapes.
 4. A thermistor as recited in claim 2, wherein said first and second electrodes and said field conducting electrode are substantially bar-shaped and said first and second electrodes are longer than said field conducting electrode.
 5. A thermistor as recited in claim 2, wherein said first and second electrodes are located at opposite edges of said first surface.
 6. A thermistor as recited in claim 5, wherein said first surface is substantially rectangular and each of said first and second electrodes extends to the other opposite edges of said first surface.
 7. A thermistor as recited in claim 2, comprising a plurality of said field conducting electrodes located between said first and second electrodes, wherein the distance between a field conducting electrode and one of said first and second electrodes is substantially different from the distance between adjacent ones of said field conducting electrodes.
 8. A thermistor as recited in claim 7, wherein the first mentioned distance is less than the second mentioned distance.
 9. A thermistor as recited in claim 2, wherein said first and second elEctrodes are substantially circular in shape.
 10. A thermistor as recited in claim 2, further comprising a conducting edge limitation located at said first surface.
 11. A thermistor as recited in claim 10, wherein said edge limitation extends substantially completely around the edges of said first surface.
 12. A thermistor as recited in claim 2, further comprising a protective layer at the parts of said first surface not covered by said first and second electrodes and said field conducting electrode.
 13. A thermistor as recited in claim 12, wherein said protective layer consists essentially of silicon dioxide.
 14. A thermistor as recited in claim 12, wherein said first surface comprises a major surface of said semiconductor body, said body further comprising a second major surface opposite said first surface and a second protective layer covering said second major surface, said second layer consisting essentially of one of silicon dioxide and aluminum oxide.
 15. A thermistor as recited in claim 2, wherein said semiconductor body consists essentially of one of intrinsic germanium, silicon, and indium-antimonide. 